Whats the difference between entering units and scaling the model at the end
The result should be the same if you scale your mesh in edit mode. However if you scale the whole object in object mode your object will behave differently when used with particles or in physics simulations. The particles and the forces on the object will be scaled as well. To avoid that problem you can apply the object-mode scale to the mesh by pressing [Ctrl+A] and selecting "Scale".